Page 131 - Handbook of Battery Materials

3.2 Tunnel Structures 99

[021] twins
micro-twin boundaries
[061] twins

Growth direction
Figure 3.6 Projection of the manganese atoms in the rams-
dellite lattice onto the bc-plane. The oxygen atoms are not
shown. The twinning planes [021] (above) and [061] (below)
are marked with arrows. The twins at these planes are gen-
erated by rotating the shaded ramsdellite cells by either 60 ◦
◦
or 120 around the a-axis. (Adapted from Ref. [41].)
kinds of disorder in the lattice, particularly when there is a large amount of micro
twinning. Although the model of Chabre and Pannetier seems to be very close to
reality, there might be some features in the structural chemistry of γ -MnO 2 which
still have to be integrated into an optimized model such as the vacancies at the
Mn 4+ sites, the presence of Mn , and the presence of larger tunnels (e.g., [1 × 3]
3+
or [2 × 2]) in the real lattice.
As already mentioned above the γ -MnO 2 /ε-MnO 2 modification is usually ob-
tained by electrochemical deposition at graphite, lead, or titanium anodes from
a solution of Mn 2+ salts in strong acids (usually H 2 SO 4 ) (for references see Ref.
[47]). Electrochemically prepared manganese dioxide (EMD) usually grows with
a fibrous texture along the [0 2 1] and [0 6 1] faces, as described above. Nsutite,
the naturally occurring modification of γ -MnO 2 , shows a similar, needle-shaped,
crystallite morphology. The fibrous habit can best be seen in chemically prepared
samples of γ -MnO 2 . These materials typically consist of bundles and small balls
of tiny needles with an average length of about 1–4 pm an average width of about
100–200 nm and an average thickness of 20–100 nm. In contrast to EMD, which
usually has a compact but porous consistency, the chemically prepared manganese
dioxides (CMDs) have a very low volumetric density and are poorly compressible.
The degree of oxidation in EMD and in CMD is lower than in pure pyrolusite
or ramsdellite. Depending on the (chemical) preparation method, the value of